The security services have identified 3,000 potential Islamist extremists in the UK, but how can they be sure who are the dangerous ‘critical few’?

The London HQ of MI5, the UK’s counter-terrorism organisation. Photograph: Frank Baron

While one set of headlines blame “MI5 blunders” for allowing “Jihadi John to slip the net”, others claim that the British security services’ past “harassment” of the Islamic State (Isis) executioner was in some way responsible for his radicalisation in the first place.

Both are potentially dangerous caricatures of the probable answer to the complex question of how a “quiet, studious” London schoolboy now named as Mohammed Emwazi could become so radicalised in modern Britain that he is now one of the world’s most wanted men.

IS militant ‘Jihadi John’ named as Mohammed Emwazi from London

So far there are only sketchy, and contradictory, accounts of how MI5 and the police came into contact with Emwazi at least a dozen times over the past six years before he left Britain for Syria to join Isis. It is not even certain that he was on a Home Office watchlist when he left in 2013, despite court papers showing he was associated with two suspects who absconded while under control orders.

Parliament’s intelligence and security committee (ISC) will, once its own troubles are sorted out, launch a detailed investigation into these issues and the allegedly botched attempt to turn Emwazi when he was first intercepted six years ago amid fears he was trying to join a Somali terrorist group.

But the case raises a question which politicians and security chiefs have been struggling with for the past two decades: how to identify and tackle suspected terrorists before they they turn to murder and how to do that within Britain’s human rights-based criminal justice system.

The problem facing the security services, as underlined by the “self-starters” or “lone wolves” in this case and the Woolwich murder of Fusilier Lee Rigby, is that they have at least 3,000 people in Britain on their databases whom they regard as potential Islamist extremists.

Many of them will be flagged as “low level subjects of interest” on the periphery perhaps of a known network or in contact with a particular individual. Attending an anti-US rally or meeting to hear an extremist preacher is not necessarily enough to get on the radar.

Perhaps 600 to 700 have gone to Syria or Iraq, and some of them have already come back. Yet at one time no more than several dozen, say 30 to 40 suspects, might be actively engaged in planning an attack in Britain or abroad.

Although MI5 now has a staff just short of 4,000 – more than double the number at the time of 9/11 – it can only hope to monitor effective 24/7 human and digital surveillance on this kind of number of high priority suspects. The question is how to identify the “critical few” when so many are moving in and out of their intelligence radar screen.

A little noticed official response to the ISC report on Woolwich published on Thursday gave a few more hints of how this prioritisation of limited intelligence resources takes place.

“MI5’s investigative and operational staff are constantly working directly on or supporting the full range of investigations: there is no pool of reserve staff available and waiting to be deployed. MI5 is able to move resources (whether staff or equipment) at very short notice to counter high priority threats and inevitably other investigations will therefore receive fewer resources as a consequence,” the Downing Street response revealed.

It also disclosed that in response to Woolwich the security service has implemented a new programme, Operation Danube, to “manage the level of risk posed by low level subjects of interest … This is now fully operational and whenever MI5 and the police close an investigation, any remaining subjects of interest are referred to this programme and the residual risk they pose is assessed.”

It also disclosed that a dedicated team now exists looking at “self-starters” who are outside the more usual terror networks and identifying recurring “subjects of interest”who feature in more than one extremist network.

But the fundamental problem remains of how to to identify those who are vulnerable to being drawn into terrorism and those whose perfectly legal extremist politics may turn into violence and terrorism.

MI5 were one of the first intelligence agencies to understand through its behavioural science unit that “there is no single pathway” to radicalisation and therefore no single action or profile that could be used to deter the potential terrorist.

This has not been used as a recipe for inaction but it has meant they have steered clear of stereotypes such as targeting only religious fundamentalists. Instead the increasingly successful Prevent programme has been used to develop anti-radicalisation initiatives such as the Channel safeguarding project which has already seen hundreds of people offered support. A referral is now regarded as the default option for the lowest-level suspects believed to be on their way to Syria.

What to do with the “critical few” once they have been identified but before they have committed any criminal acts in Britain has been a problem ever since 2001.

Different solutions have been tried in the past 15 years such as the unlawful Belmarsh indefinite detention regime, control orders and latterly terrorism prevention and investigation measures. All were alternatives to mass internment and none have involved monitoring more than 30 to 40 terror suspects at any one time. They remain but one tool in a ever growing array of counter-terrorist measures in the hands of the authorities.